An isolated rat brain preparation will be perfused with different agents (methadone, morphine, chlorpromazine and others). The effects on the metabolic activity of the brain will be evaluated by chemical analyses of perfusion fluid and brain tissue. Electrical activity will be recorded from stereotaxically placed electrodes in discrete areas of the brain of the intact rat and in the isolated, perfused brain. Similar experiments will be done with rats that have been treated with addicting drugs until they have become tolerant to the effects of the drug.